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Fun with Muons, GPS, Radar, Etc FunWithMuons051607b.doc (From FunWithMuons093904.doc and Fun_LHL.pdf) Fun With Muons, GPS, Radar, etc. Lee Historical Lecture Harvard University March 18, 2003 R.L. Garwin Thank you for the honor of presenting the Lee Historical Lecture. In thinking about this opportunity, I considered two candidates-- first the 1957 demonstration that parity is not conserved-- that a mirror universe can readily be distinguished in the case of the mu meson. The second possibility would be to present my involvement in observations from 1950 to the present regarding defense against ballistic missiles. I chose the parity experiment, but I would be glad to take questions at the end on any subject in which I have been involved. Foil 1 I begin with a wonderful experiment I had the good fortune to conduct in January 1957 with Leon Lederman and Marcel Weinrich at the Nevis Cyclotron Laboratory of Columbia University. I want to tell briefly the story of that experiment and another which followed it, and then to describe a couple of the simple techniques involved, which I had introduced years earlier to particle physics. March 18, 2003 Fun With Muons, GPS, Radar, etc. 1 Harvard University Richard L. Garwin 08/30/2007 Foil 2 I then leave physics for technology and policy (both in my talk and in my personal history) and touch lightly on a few topics simple in concept and incredibly complex in implementation with which I was involved. First the muons. Muons first came on the scene (as "mesotrons" because their mass of 207 electron masses was intermediate between that of the electron and the proton) in the 1930s as the penetrating component of the cosmic radiation. Copiously produced by protons interacting with air nuclei, muons were strangely diffident about nuclear interactions themselves. Only their mass fit the requirement of the Yukawa meson. The puzzle was solved with the discovery of the pi meson or pion, which in the fullness of time (20 nanoseconds) decays to the muon, which itself has only a weak interaction, in addition to the electromagnetic interaction of its electric charge. Cosmic ray muons contribute significantly to the radiation dose received by humans at sea level-- about twice as much annually as the dose received from the potassium-40 radioactivity built into our bodies with the stardust from which we are all made. Columbia University Physicist I.I. Rabi famously said about the muon, "Who ordered that?" In the summer of 1956, T.D. Lee and C.N. Yang explored seriously the observable consequences of violation of the laws of conservation of parity and of charge conjugation in weak decays, as would potentially explain the ability of the theta meson to decay into two particles and the tau meson into three particles-- no problem-- unless the tau and the theta were the same, as seemed increasingly likely. To their surprise, they found that although it seemed entirely feasible to investigate parity violation in the case of ordinary radioactive materials, such experiments had not been done. In fact, I recall my mentor, Enrico Fermi, probably around 1950 telling me that he had heard from Ed Purcell of his experiment (with Norman Ramsey) to detect the electric dipole moment of the neutron, which Fermi explained would have to be zero if parity were a good quantum number. Lee and Yang in their theoretical publication of summer 1956 showed that if a sample of polarized Co-60 nuclei emitted more electrons along the polarizing magnetic field than in the opposite direction (or vice versa), this would be proof of parity nonconservation. They also calculated that parity violation in the decay of the positive pion would lead to a polarization of the muon along its direction of travel, and that if the muon spin March 18, 2003 Fun With Muons, GPS, Radar, etc. 2 Harvard University Richard L. Garwin 08/30/2007 direction were retained while the muon stopped in photographic emulsion about a millimeter away, the decay electrons from the muon decay (two microseconds later) might similarly be more numerous along the direction of flight than in the opposite direction, or vice versa. Prof. Chien-Shiung Wu of Columbia University in the spring of 1956 resolved to do the Co-60 experiment and that summer recruited Ernest Ambler and his colleagues at the National Bureau of Standards in Washington, DC. Many groups began looking at pi-mu-e decay in photographic emulsion, where it was all too easy to find "effects" caused by scanning bias. To make a long story short, on Friday, January 4, 1957 when I returned from my IBM day leading a program with about 100 people to build a superconducting computer based on thin-film cryotrons, I received a call from my good friend and Columbia physics colleague, Leon Lederman, with news that Ambler, Wu, and company were obtaining positive results. Leon and I agreed to meet at the Nevis Cyclotron of Columbia University in 15 minutes, at 8 p.m. Necessity (and even adversity) being the mother of invention, we benefited greatly from the fact that the machine shop and the stockroom were closed and that the cyclotron would begin its weekend rest Saturday morning. Leon and his graduate student Marcel Weinrich had a setup in the external beam of the cyclotron, investigating the decay of muons, plus and minus. The muons were stopped in a block of graphite or other material to be investigated, and the electron decay product of each muon detected as a function of time after the muon(s) stopped. The muons originated from the decay in flight of pions copiously produced in an internal target in the cyclotron by a 400 MeV proton beam. The 85-MeV pions brought out from the cyclotron vacuum in a thin window have a range of about five inches of graphite. Eight inches of graphite thus blocked the pions and put the peak of the muon stopping distribution in the target under investigation. The counting rate in the electron telescope was typically 20 per minute for mu-plus and about 100 per minute for mu-minus with a background of about one count per minute. The arrangement that Lederman and Weinrich had used is shown in the first figure. Ignore the coil! March 18, 2003 Fun With Muons, GPS, Radar, etc. 3 Harvard University Richard L. Garwin 08/30/2007 Foil 31 Leon's brilliant idea that night was to depart from the suggestion to study muons originating from stopped pions, that would be polarized along their direction of flight. Their range was less than a millimeter, and so it was difficult to separate a population of polarized muons from the stopped pions, which had a much broader distribution in range. Our beam of cyclotron muons, however, originated from the decay in flight of pions. Muons emitted forward would have the velocity of the pion in its rest frame plus (roughly speaking, since the velocities add relativistically) the relatively small additional velocity of a muon which is born with an energy of 4 MeV in the rest frame of the pion. The forward emitted muons have greater momentum than the pions, and those emitted backward have less, so that the forward-emitted muon "contamination" of the pion beam would be strongly polarized if parity conservation were maximally violated. So it remained only to try to measure the relative numbers of electrons in different directions from the stopped muons. Easier said than done, because we could not stay in the beam room because of radiation background, and our electronics were 150 meters away (and 30-m difference in altitude). We would need to shield the muons from the fringe field of the cyclotron so that their (unknown) magnetic moment would not lead to precession during their decay lifetime, and we would need to move a counter telescope around the muon population. Furthermore, electrons emitted in different directions would traverse different amounts of the stopping block, so that the expected small asymmetry might be masked by such dirt effects. In my work at the IBM Laboratory at Columbia University, I had been studying liquid and solid helium 3 for several years, by the methods of spin echoes. I judged that we could make a virtue of necessity and apply a uniform magnetic field to the muon population so March 18, 2003 Fun With Muons, GPS, Radar, etc. 4 Harvard University Richard L. Garwin 08/30/2007 that any asymmetry in decay probability would precess with the muon spin, in the horizontal plane. Within a couple of hours we had fashioned from scrap a lucite cylinder surrounding the graphite stopping block, and a coil of fine enameled copper wire wound on the cylindrical lucite coil form. We started taking data with different values of applied current (hence precessing field) and found substantial effects on counting rate as a function of coil current for the telescope set at a fixed position. Unfortunately, when the cyclotron shut down for the weekend at 9 a.m. Saturday, the effect seemed to have vanished. Returning to the beam room, we found that the copper wire had overheated and expanded and was lying at the base of the coil form, no longer serving its function of precessing muon spins. Monday was maintenance day at the cyclotron so we were runnng again only Monday night, this time with a rectangular solenoid wound directly on the graphite stopping block. I had chosen graphite for positive muons, because its low atomic number would minimize the scattering of the decay electrons, and its quasi-metallic nature would prevent the formation of muonium, which would subject the muon to the enormous magnetic field of its partner electron.
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